Identification and visualization of gaps between cardiac ablation sites

ABSTRACT

A method includes receiving locations of multiple ablation sites formed on a surface of a heart. Distances are measured among at least some of the ablation sites based on the locations. One or more gaps between the ablation sites, which meet an alerting criterion, are identified. The identified gaps are indicated to an operator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of pending U.S. patent applicationSer. No. 14/293,400, filed Jun. 2, 2014.

FIELD OF THE INVENTION

The present invention relates generally to cardiac ablation, andparticularly to methods and systems for mapping cardiac ablation sites.

BACKGROUND OF THE INVENTION

Radio-Frequency (RF) ablation is a common procedure for treating variouscardiac disorders. Various ablation techniques, and methods forvisualizing the ablation procedure, are known in the art. For example,U.S. Patent Publication 2013/0116881, whose disclosure is incorporatedherein by reference, describes a system which provides heart ablationunit control. The system includes an input processor for acquiringelectrophysiological signal data from multiple tissue locations of aheart and data indicating tissue thickness at the multiple tissuelocations. A signal processor processes the acquiredelectrophysiological signal data to identify location of particulartissue sites of the multiple tissue locations exhibiting electricalabnormality in the acquired electrophysiological signal data anddetermines an area of abnormal tissue associated with individual sitesof the particular sites. An ablation controller automatically determinesablation pulse characteristics for use in ablating cardiac tissue at anindividual site of the particular tissue sites in response to theacquired data indicating the thickness of tissue and determined area ofabnormality of the individual site.

U.S. Pat. No. 7,001,383, whose disclosure is incorporated herein byreference, describes a method for ablating tissue in a heart of asubject during an ablation procedure. The method includes applying alocal treatment to the heart at a plurality of sites designated forablation. At each respective site, a parameter is sensed that isindicative of a level of ablation at the site. The method preferablyincludes displaying a map of the heart, and designating, on the map,during the ablation procedure, indications of the respective levels ofablation at the sites, responsive to the respective sensed parameters.

U.S. Patent Publication 2008/0172049, whose disclosure is incorporatedherein by reference, describes an apparatus and method for ablatingtissue in a heart of a subject during an ablation procedure. The methodincludes contacting an ablation catheter tip to tissue of the heart at aplurality of sites designated for ablation; sensing at each respectivesite a feedback signal from the ablation catheter indicative of successof the intended local ablation; storing any available data defining acurrent position of the ablation catheter tip relative to the heart at amoment of sensing the feedback signal indicative of a failed intendedablation for later re-visit; displaying a map of a region of interest ofthe heart; and designating, on the map display, indications of the sitescorresponding to when the required electrical current is above thethreshold current value indicative of a gap in an ablation line or ring.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa method including receiving locations of multiple ablation sites formedon a surface of a heart. Distances among at least some of the ablationsites are measured based on the locations. One or more gaps between theablation sites, which meet an alerting criterion, are identified. Theidentified gaps are indicated to an operator.

In some embodiments, identifying the gaps includes detecting the gapsthat are larger than a first threshold but smaller than a secondthreshold. In other embodiments, measuring the distances includesscaling a distance between first and second ablation sites by a scalingfactor that depends on an ablation quality associated with one or bothof the first and second ablation sites.

In some embodiments, measuring the distances includes clustering theablation sites into groups by connecting adjacent ablation sites whosedistances are smaller than a first threshold, and identifying the gapsincludes identifying separations between groups that are smaller than asecond threshold. In other embodiments, clustering the ablation sitesincludes iteratively calculating the distances from a given cluster toone or more of the ablation sites, and adding an ablation site to thegiven cluster upon finding that a distance from the ablation site to thegiven cluster is smaller than the first threshold.

In some embodiments, measuring the distances includes assessing thedistances depending on parameters of an ablation signal used for formingthe ablation sites. In other embodiments, measuring the distancesincludes assessing the distances depending on sizes of the ablationsites.

There is also provided, in accordance with an embodiment of the presentinvention, a system including an interface and a processor. Theinterface is configured to receive locations of multiple ablation sitesformed on a surface of a heart. The processor is configured to measuredistances among at least some of the ablation sites based on thelocations, to identify one or more gaps between the ablation sites thatmeet an alerting criterion, and to indicate the identified gaps to anoperator.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a system forcardiac ablation, in accordance with an embodiment of the presentinvention;

FIG. 2A is a diagram illustrating a heart undergoing ablation, inaccordance with an embodiment of the present invention;

FIG. 2B is a diagram illustrating annotated ablation sites on an imageof a heart, in accordance with an embodiment of the present invention;

FIGS. 3A-3E are diagrams illustrating a method for detecting andvisualizing ablation gaps, in accordance with an embodiment of thepresent invention; and

FIG. 4 is a flow chart illustrating a method for detecting andvisualizing ablation gaps, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Cardiac ablation is a procedure that locally heats and ablates cardiactissue on the inner surface of a heart cavity so as to relieve cardiacdysfunction. As a physician, typically a cardiologist, performs theablation therapy, the physician typically forms ablation lesions byapplying RF energy, for example, to the heart tissue using an ablationelectrode positioned at a distal end of a catheter. The ablationelectrode contacts the endocardium in the heart cavity at multiplediscrete ablation sites along a predefined trajectory.

The cardiologist may monitor the procedure by observing the position ofthe catheter tip in an image of the heart on a display. The catheter tipposition can be detected, for example, by a catheter position trackingsystem or imaging system.

If the cardiologist creates adjacent ablation lesions that are too farapart, the resulting gap may not completely eliminate the parasiticelectrical pathways of the cardiac activation wave, for example, and thecardiac dysfunction may not be completely alleviated.

Embodiments of the present invention described herein provide methodsfor identifying and visualizing gaps between cardiac ablation sites. Insome embodiments, a processor of a cardiac mapping and ablation systemreceives the coordinates of multiple ablation sites on the surface ofthe heart. The processor then identifies intolerable gaps betweenablation sites, e.g., gaps that are larger than a certain threshold. Theprocessor presents the identified gaps, so as to enable the physician toeliminate them.

In some embodiments, the processor identifies the intolerable gaps usingan iterative process that measures distances between ablation sites andprogressively clusters ablation sites into groups. The process typicallypresents the resulting groups or clusters, and emphasizes anyintolerable gaps found between them.

Using the disclosed technique, the physician is provided with a clearreal-time visual display that highlights locations where ablationquality is likely to be insufficient. Using such a display, thephysician is able to revisit the locations in question and complete theablation procedure successfully.

System Description

FIG. 1 is a block diagram that schematically illustrates a system 20 forcardiac ablation, in accordance with an embodiment of the presentinvention. System 20 comprises a probe 22, in the present example acardiac catheter, and a control console 24. In the embodiment describedherein, it is assumed by way of example that catheter 22 is used for theablation of tissue in a heart 26 in a patient 28 using an ablationelectrode positioned near a distal end 40 of catheter 22. Alternativelyor additionally, catheter 22 may be used any other suitable diagnosticand/or therapeutic procedure such as electro-physiological (EP) cardiacsignal mapping of a cavity of heart 26 of patient 28 for the diagnosisof cardiac dysfunctions (not shown here).

Console 24 comprises a processor 42, typically a general-purposecomputer, with suitable front end circuitry for receiving signals fromprobe 22 via an interface 38 and for controlling the other components ofsystem 20 described herein. Processor 42 may be programmed in softwareto carry out the functions that are used by the system, and theprocessor stores data for the software in a memory 50. The software maybe downloaded to console 24 in electronic form, over a network, forexample, or it may be provided on non-transitory tangible media, such asoptical, magnetic or electronic memory media. Alternatively, some or allof the functions of processor 42 may be carried out by dedicated orprogrammable digital hardware components.

An operator 30, typically a physician or cardiologist, inserts catheter22 into patient 28 and navigates the catheter through the patient'svascular system. Cardiologist 30 moves distal end 40 of catheter 22 inthe vicinity of the target region in heart 26 for ablation.

First, relating to sensing and recording the position (i.e. coordinates)of the ablation electrode at distal end 40 in patient 28 during ablationtherapy, a position sensing system is may be used to measure theposition of distal end 40 of catheter 22 in the heart cavity in someembodiments. Console 24 comprises a driver circuit 34, which drivesmagnetic field generators 36 placed at known positions external topatient 28, e.g., below the patient's torso.

A magnetic field sensor, typically comprising coils (not shown), isattached to catheter 22 near distal end 40. The position sensorgenerates electrical position signals in response to the magnetic fieldsfrom the coils, thereby enabling processor 42 to determine thecoordinates, or position, of distal end 40 within the heart cavity, andthus the coordinates of the ablation electrode.

In other embodiments, system 20 may use impedance-based position sensingtechniques (e.g., advanced catheter location (ACL) technologies) todetermine the position of distal end 40 within the heart cavity. System20 in these embodiments is configured to drive current between at leastone current electrode at distal end 40 and a plurality of body surfaceelectrodes on patient 28 (not shown in FIG. 1) typically attached to thepatient's chest above the heart. Processor 42 then determines theposition of the distal end based on the measured currents between theplurality of body surface electrodes and the at least one currentelectrode at distal end 40. Further alternatively, system 20 maydetermine the position of distal end 40 (and thus of the ablationelectrode) in any other suitable way.

Relating to RF ablation, console 24 also comprises an RF signalgenerator, which is used to apply an RF signal to the ablation electrodeat distal end 40 of catheter 22. When the electrode contacts the hearttissue, the RF signal locally heats and induces a local necrosis of theheart tissue at the ablation site. The position sensing system, or animaging system such as ultrasound, fluoroscopy, or magnetic resonanceimaging (MRI), for example, records the position of the multipleablation sites formed by the ablation electrode during the procedure.

Processor 42 displays an image 44 of heart 26 with the recordedpositions of the multiple ablation sites, possibly overlaid with localelectro-cardiac signal measurements on the simulated surface, tocardiologist 30 on a display 46.

Interface 38 is configured to relay the coordinates of the multipleablation sites formed by the ablation electrode to processor 42. In someembodiments, the interface may be configured to receive signals from themagnetic field sensor signals indicative of the coordinates of theablation electrode positioned near distal end 40 of catheter 22.Processor 42 then computes the position of distal end 40 (e.g., theposition coordinates of the ablation electrode).

In other embodiments, the interface may be configured to receive thecoordinates of the ablation sites recorded by any suitable imagingsystem (e.g., ultrasound, fluoroscopy, MRI, etc.). Processor 42 mayreceive the coordinates of the multiple ablation sites by any suitablemethod in order to use the coordinates to identify ablation gaps as perthe embodiments described herein.

Finally, system 20 may also comprise EP cardiac signal mapping, whichmay be used to assess the effectiveness of the ablation therapy in realtime. Catheter 22 may also comprise one or more mapping electrodes nearthe catheter distal end to measure electro-cardiac signals at one ormore respective contact points with the heart tissue. Processor 42 usesthe position of distal coordinates of the map points to construct asimulated surface of the cardiac cavity, or chamber, in question.Processor 42 then combines the electrical potential measurements of themap points with the simulated surface to produce a map of the potentialsoverlaid on the simulated surface. System 20 may use fluoroscopy, ormagnetic resonance imaging (MRI), for example, to synchronize images ofthe heart with the EP mapping in the catheter position sensing system.

This method of position sensing is implemented, for example, in theCARTO™ system, produced by Biosense

Webster Inc. (Diamond Bar, California) and is described in detail inU.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and6,332,089, in PCT Patent Publication WO 1996/05768, and in U.S. PatentApplication Publications 2002/0065455, 2003/0120150 and 2004/0068178,whose disclosures are all incorporated herein by reference. The VisiTag™module, produced by Biosense Webster Inc. (Diamond Bar, California),provides a visual representation of the ablation lesions to assist thecardiologist in ablation strategy by displaying different parameters ofthe lesion formation.

The embodiments shown in FIG. 1 are merely for visual clarity and not byway of limitation of the embodiments of the present invention. System 20is not limited to RF ablation, which is used throughout as an exampleherein. Any other suitable cardiac ablation therapy, such as focusedlaser ablation or ultrasound ablation may be used. Catheter 22 is notlimited to one ablation electrode positioned at distal end 40, but maycomprise any suitable number of ablation electrodes positioned at anysuitable positions along the body of catheter 22. The catheter maycomprise, for example a lasso catheter having multiple ablationelectrodes distributed along its distal end.

Identifying Gaps in the Cardiac Ablation Sites

During the ablation procedure, cardiologist 30 typically ablates thecardiac tissue discretely, site-by-site, using the ablation electrode.In some cases, cardiologist 30 may miss a region in the cardiac tissueduring ablation along the planned spatial trajectory of the ablationsites, the missed region referred to herein as a gap. If gaps arepresent, the cardiac dysfunction may not be alleviated by the procedure.Hence, identifying gaps between the ablation lesions and alerting thecardiologist are highly beneficial in assisting the cardiologist inassessing the overall effectiveness of the ablation procedure, andimproving it as needed.

FIG. 2A is a diagram illustrating heart 26 undergoing ablation, inaccordance with an embodiment of the present invention. The ablationelectrode at distal end 40 (shown in black in FIG. 2A) of catheter 22contacts the heart cavity at multiple ablation sites 100 to induce localnecrosis of the heart tissue. A lesion is formed at each ablation site100.

Using position tracking of a position sensor at distal end 40, orimaging systems as described previously, processor 42 records thepositions of multiple ablation sites 100 as cardiologist 30 forms themultiple lesions on the surface of the heat cavity with the ablationelectrode. The position of ablation sites 100 can be displayed tocardiologist 30 in real time on image 44 of heart 26 on display 46. EPmapping electrodes near the distal end of catheter 22 or on a separatecatheter (not shown in FIG. 1) can be used to monitor changes in theelectro-cardiac signals measured at the one or more mapping electrodesas described previously in response to the ablation therapy.

FIG. 2B is a diagram illustrating annotated ablation sites 100 on image44 of a heart 26, in accordance with an embodiment of the presentinvention. FIG. 2B shows an enlarged view of the ablation region in FIG.2A in an inset 90 of ablation sites 100 after processing by processor42. An example algorithm for producing this view is described in FIGS.3A-3E below.

The example of FIG. 2B shows ablation sites 100 as circles. Some of theablation sites are connected by lines 120. Each group of ablation sites100 that are interconnected by lines 120 is referred to as a site groupor cluster. Intolerable gaps 110 between ablation sites are marked inFIG. 2B with bold lines 125 (and highlighted accordingly to thephysician).

In the present embodiment, an intolerable gap is defined as a separationbetween ablation sites or clusters that is larger than an AdjacentDistance Threshold (ADT) but smaller than an Out Of Group (OOG)threshold. The rationale behind this dual threshold scheme is that verylarge gaps (>OOG) are likely to be intentional. As such, separationsthat are larger than the OOG threshold are not considered intolerablegaps, and are typically not highlighted to the physician. An example ofsuch a separation is shown in the figure as a bracket 115.

The diagrams of FIGS. 2A and 2B are depicted merely for conceptualclarity and not by way of limitation of the embodiments of the presentinvention. The lengths of ADT bracket 110 and OOG bracket 115 (e.g., thevalues of these parameters) are merely by way of example, and can bechosen by the cardiologist to be any suitable value. Typical values ofADT and OOG (e.g., ADT bracket 110 and OOG bracket 115) are 5 mm and 20mm, respectively.

Note that in FIG. 2B, the diameters of the lesions at multiple ablationsites 100 formed by the ablation electrode at distal end 40 of catheter22 are shown to be roughly of the same diameter, for the sake ofclarity. In alternative embodiments, the diameters of ablation sites 100may differ from one another, for example depending on the ablationparameters (e.g., ablation time or ablation signal power) set for eachablation site.

In some embodiments, the distances measured between ablation sites 100(e.g., lines 120) do not consider the site diameter. For example, thedistances may be computed between the ablation site centers. In otherembodiments, the distances measured between ablation sites 100 depend onthe site diameters. For example, for the same ablation site centers, thedistance between large-diameter lesions are smaller than the distancebetween small-diameter lesions.

In some embodiments, processor 42 scales the measured physical distancebetween ablation sites by a scaling factor that depends on the ablationquality (also referred to as ablation index) of one or both of theablation sites. As a result, low-quality ablation sites will beinterpreted as being further apart than high-quality sites.

FIGS. 3A-3E are diagrams illustrating an algorithmic flow for detectingand visualizing ablation gaps, in accordance with an embodiment of thepresent invention. In FIG. 3A, processor 42 receives and registers thepositions of multiple ablation sites 100.

In the first step of the algorithm flow, processor 42 examines a certainregion 130. For each ablation site 100, the processor searches for theclosest neighbor ablation site which does not have a closer neighbor. Ifthe distance between the two ablation sites is smaller than the adjacentdistance threshold (ADT), processor 42 flags them as pairs. Thesedistances are connected with lines 120 as shown in FIG. 3B. At the endof the first step, all of ablation sites 100 are either grouped intoconnected pairs or remain unpaired. Note that this step does notinherently mark every pair of ablation sites that are closer than theADT threshold.

In the second step of the algorithm, processor 42 connects the clusters(including unpaired individual sites 100 that are regarded assingle-site clusters) that are closer to one another than the ADTthreshold. This step is shown in FIG. 3C. In this step, some of thedistances 155 (between a single-site cluster and a multi-site cluster)are computed between sites 100 and intermediate points 145 on lines 120.This clustering process typically continues until reaching stability,i.e., until it is impossible to find new pairs of clusters to connect.The algorithm step of FIG. 3C produces multiple clusters 140 that areseparated from one another by at least the ADT threshold (sinceotherwise they would have been connected).

In the third and last step, shown in FIG. 3D, processor 42 identifiesgaps 110 between adjacent clusters 140, which are smaller than the OOGthreshold. The identified gaps are marked with bold lines or otherwisehighlighted on display 46 as intolerable gaps 125.

Gaps between ablation site clusters that are separated by more than theOOG threshold are typically not marked and not considered intolerablegaps. FIG. 3E demonstrates a scenario of this sort. The figure shows tworegions 130 and 160. The separation between the nearest clusters in thetwo regions (marked as a gap 115) is larger than the OOG threshold. Assuch, gap 115 is not marked and not considered intolerable.

The diagrams shown in FIGS. 3A-3E are depicted merely for conceptualclarity in illustrating the disclosed techniques, and not by way oflimitation of the embodiments of the present invention. In alternativeembodiments, processor 42 may use any other suitable algorithm foridentifying gaps 125. For example, any suitable reference points basedon the coordinates of multiple ablation sites 100 can be used incomputing distances between adjacent ablation sites for identifyinggaps. The disclosed techniques are not limited to the center-to-centerdistance or intermediate points 145 as described previously.

FIG. 4 is a flow chart illustrating a method for detecting the presenceof ablation gaps, in accordance with an embodiment of the presentinvention. The method begins with physician 30 performing ablation atmultiple ablation sites 100 along a desired trajectory on the innersurface of heart 26, at an ablation step 200. At a location input step204, processor 42 receives via interface 38 the locations (e.g.,coordinates) of ablation sites 100. As noted above, the locations ofablation sites 100 may be received from any suitable source, such asfrom a magnetic position tracking system or an imaging (e.g.,ultrasound) system.

At a distance measurement step 208, processor 42 measures the distancesbetween ablation sites and/or site clusters, as demonstrated by FIGS.3A-3E above. At a site displaying step 208, processor 42 displays theclustered ablation sites on display 46.

At a gap checking step 216, processor 42 checks for the presence ofintolerable gaps between the clustered ablation sites. In the presentexample, processor 42 checks whether any of the gaps is larger than theADT threshold but smaller than the OOG threshold. Alternatively,however, any other suitable alerting criterion can be used foridentifying a gap as intolerable.

If no intolerable gaps were found, the method loops back to step 200above. If one or more gaps were found to be intolerable, processor 42marks the identified gaps on display 46, at a marking step 220. Anysuitable visual means can be used for this purpose. The method thenloops back to step 200 above, in which the physician optionally formsadditional ablation sites 100 in the identified gaps.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and sub-combinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art. Documents incorporated by reference in the present patentapplication are to be considered an integral part of the applicationexcept that to the extent any terms are defined in these incorporateddocuments in a manner that conflicts with the definitions madeexplicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

1-14. (canceled)
 15. A method, comprising: receiving locations of firstand second ablation sites formed on a surface of a heart; measuring agap between first and second ablation sites based on the locations; andindicating the gap on a display to an operator; wherein when the gap isbeneath an adjacent distance threshold, the step of indicating comprisesindicating that the first and second ablation sites are connected;wherein when the gap is above the adjacent distance threshold andbeneath an out of group threshold, the step of indicating compriseshighlighting the gap between the first and second ablation sites; andwherein when the gap is above the out of group threshold, the step ofindicating comprises indicating that the gap between first and secondablation sites is intentional.
 16. The method according to claim 15,wherein indicating that the first and second ablation sites areconnected comprises showing a line connecting the first and secondablation sites.
 17. The method according to claim 15, whereinhighlighting a connection between the first and second ablation sitescomprises showing a bold line connecting the first and second ablationsites.
 18. The method according to claim 15, wherein indicating that thegap between first and second ablation sites is intentional comprisesshowing no connection between the first and second ablation sites. 19.The method according to claim 15, wherein measuring the gap comprisesmeasuring the distance between the first and second ablation sites. 20.The method of claim 19, wherein the adjacent distance threshold is adistance of 5 mm and the out of group threshold is a distance of 20 mm.21. The method according to claim 15, wherein measuring the gapcomprises scaling a distance between the first and second ablation sitesby a scaling factor that depends on an ablation quality associated withone or both of the first and second ablation sites.
 22. The methodaccording to claim 15, further comprising: clustering a plurality ofablation sites into at least a first and second group based on themeasured gap; and determining closest neighbor ablation sites betweenthe first and second group, the first ablation site being the closestneighbor in the first group and the second ablation site being theclosest neighbor in the second group.
 23. The method according to claim15, further comprising: receiving a location of a third ablation siteformed on a surface of a heart; and measuring a gap between anintermediate location between the first and second ablation sites andthe location of the third ablation site; wherein when the gap is beneatha adjacent distance threshold, the step of indicating comprisesindicating that the first, second and third ablation sites areconnected; wherein when the gap is above the adjacent distance thresholdand beneath a out of group threshold, the step of indicating comprisesindicating that the gap between the third ablation site and theintermediate location is intolerable; and wherein when the gap is abovethe out of group threshold, the step of indicating comprises indicatingthat the gap between third ablation site and the intermediate locationis intentional.
 24. A system, comprising: an interface configured toreceive locations of multiple ablation sites formed on a surface of aheart; a processor configured to measure a gap between first and secondablation sites based on the locations; and a display configured toindicate the identified gaps to an operator; wherein when the processordetermines that the measured gap is beneath a adjacent distancethreshold, the display is configured to indicate that the first andsecond ablation sites are connected; wherein when the processordetermines that the measured gap is above the adjacent distancethreshold and beneath an out of group threshold, the display isconfigured to indicate that the gap between the first and secondablation sites is intolerable; and wherein when the processor determinesthat the measured gap is above the out of group threshold, the displayis configured to indicate that the gap between first and second ablationsites is intentional.
 25. The system according to claim 24, wherein thedisplay is configured to show a line connecting the first and secondablation sites to indicate that the first and second ablation sites areconnected.
 26. The system of claim 24, wherein the display is configuredto highlight the gap between the first and second ablation sites byshowing a bold line connecting the first and second ablation sites. 27.The system of claim 24, wherein the display is configured to show noconnection between the first and second ablation sites to indicate thatthe gap between first and second ablation sites is intentional.
 28. Thesystem of claim 24, wherein the processor measures the gap by measuringthe distance between the first and second ablation sites.
 29. The systemof claim 28, wherein the adjacent distance threshold is a distance of 5mm and the out of group threshold is a distance of 20 mm.
 30. The systemaccording to claim 24, wherein the processor measures the gap by scalinga distance between the first and second ablation sites by a scalingfactor that depends on an ablation quality associated with one or bothof the first and second ablation sites.
 31. The system according toclaim 24, wherein the processor is further configured to: cluster aplurality of ablation sites into at least a first and second group basedon the measured gap; and determine closest neighbor ablation sitesbetween the first and second group, the first ablation site being theclosest neighbor in the first group and the second ablation site beingthe closest neighbor in the second group.
 32. The system according toclaim 24, wherein the processor is further configured to: receive alocation of a third ablation site formed on a surface of a heart; andmeasure a gap between an intermediate location between the first andsecond ablation sites and the location of the third ablation site;wherein when the gap is beneath a adjacent distance threshold, thedisplay is configured to indicate that the first, second and thirdablation sites are connected; wherein when the gap is above the adjacentdistance threshold and beneath a out of group threshold, the display isconfigured to indicate that the gap between the third ablation site andthe intermediate location between the first and second ablation sites isintolerable; and wherein when the gap is above the out of groupthreshold, the display is configured to indicate that the gap betweenthird ablation site and the intermediate location between the first andsecond ablation sites is intentional.